A hologram recorder records a hologram in a hologram recording medium by interference of a recording beam irradiated at a predetermined incident angle to the medium via a spatial light modulator incliningly in a predetermined direction with a reference beam irradiated incliningly in an opposite direction to the recording beam so that the beams crosses at a predetermined crossing angle. The recorder includes a beam modulator for converting a gaussian-beam having a gaussian-distribution of intensity into a parallel beam having an intensity distribution tending to be uniform and allowing the parallel beam to travel to the spatial modulator as the recording beam. The gaussian-beam has a center axis decentered from an optical axis of the beam modulator in a decentering direction including a directional component of the predetermined direction in which the recording beam is inclined or an opposite directional component thereto.
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1. A hologram recorder for recording a hologram in a hologram recording medium by interference of a recording beam with a reference beam, the recording beam being irradiated at a predetermined incident angle to the hologram recording medium via a spatial light modulator in a state that the recording beam is inclined in a predetermined direction, the reference beam being irradiated in a state that the reference beam is inclined in an opposite direction to the recording beam so that the reference beam crosses with the recording beam at a predetermined crossing angle, the hologram recorder comprising:
a beam modulator for converting a gaussian-beam having an intensity distribution of a gaussian-distribution into a parallel beam having an intensity distribution tending to be uniform and allowing the parallel beam to travel to the spatial light modulator as the recording beam,
wherein the gaussian-beam has a center axis decentered from an optical axis of the beam modulator in a decentering direction including a directional component of the predetermined direction in which the recording beam is inclined or an opposite directional component to the predetermined direction.
5. A hologram recording method for recording a hologram in a hologram recording medium by interference of a recording beam with a reference beam, the recording beam being irradiated at a predetermined incident angle to the hologram recording medium via a beam modulator and a spatial light modulator in a state that the recording beam is inclined in a predetermined direction, the reference beam being irradiated in a state that the reference beam is inclined in an opposite direction to the recording beam so that the reference beam crosses with the recording beam at a predetermined crossing angle, the method comprising the steps of:
converting a gaussian-beam having an intensity distribution of a gaussian-distribution into a parallel beam having an intensity distribution tending to be uniform, and
allowing the parallel beam to travel from the beam modulator to the spatial light modulator as the recording beam,
wherein the center axis of the gaussian-beam is decentered from an optical axis of the beam modulator in a decentering direction including a directional component of the predetermined direction in which the recording beam is inclined or an opposite directional component to the predetermined direction.
2. The hologram recorder according to
3. The hologram recorder according to
4. The hologram recorder according to
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This application is a Continuation of International Application No. PCT/JP2006/309697, filed May 16, 2006.
The present invention relates to a hologram recorder and a hologram recording method for recording holograms by means of interference between a recording beam and a reference beam.
A conventional hologram recording method is disclosed in Patent Document 1. In the method disclosed therein, a reference beam (baseline beam) is applied to a hologram recording medium from a predetermined oblique direction at the incident angle of 35 degrees whereas a recording beam (object beam) is applied from an opposite oblique direction with respect to the direction in which the reference beam is made oblique, at about 90 degrees. The reference beam has a beam intensity modulated by e.g. an ND (Neutral Density) filter. Specifically, in the conventional hologram recording method, since the recording beam is applied obliquely to the hologram recording medium, apodization is employed for uniformalizing the beam intensity distribution of the Gaussian-beam as the reference beam on the Fourier plane, whereby SN ratio is improved.
Patent Document 1: JP-A-2003-43904
However, in the beam intensity uniformalization by apodization, the beam transmissivity is reduced partially, which results in a large light loss and this low light utilization efficiency has been a disadvantage in comparison with the cases where apodization is not employed.
The present invention has been proposed under the above-described circumstances. An object of the present invention is to provide a hologram recorder and a hologram recording method for hologram recording capabilities without reduction in the light utilization efficiency.
In order to solve the above-described object, the present invention makes use of the following technical means.
According to a first aspect of the present invention, there is provided a hologram recorder for recording a hologram in a hologram recording medium by interference of a recording beam with a reference beam, the recording beam being irradiated at a predetermined incident angle to the hologram recording medium via a spatial light modulator in a state that the recording beam is inclined in a predetermined direction, the reference beam being irradiated in a state that the reference beam is inclined in an opposite direction to the recording beam so that the reference beam crosses with the recording beam at a predetermined crossing angle, the hologram recorder comprising a beam modulator for converting a Gaussian-beam having an intensity distribution of a Gaussian-distribution into a parallel beam having an intensity distribution tending to be uniform and allowing the parallel beam to travel to the spatial light modulator as the recording beam, wherein the Gaussian-beam has a center axis decentered from an optical axis of the beam modulator in a decentering direction including a directional component of the predetermined direction in which the recording beam is inclined or an opposite directional component to the predetermined direction.
Preferably, an amount of decentering of the Gaussian-beam is selected in accordance with a cosine law of illumination for the recording beam irradiated to the hologram recording media.
Preferably, the amount of decentering of the Gaussian-beam is selected in accordance with the polarization direction of the recording beam with respect to the hologram recording medium as an additional factor.
Preferably, the amount of decentering of the Gaussian-beam is selected in accordance with a diffraction efficiency of the recording beam and the reference beam with respect to the hologram recording medium as an additional factor. According to a second aspect of the present invention, there is provided a hologram recording method for recording a hologram in a hologram recording medium by interference of a recording beam with a reference beam, the recording beam being irradiated at a predetermined incident angle to the hologram recording medium via a beam modulator and a spatial light modulator in a state that the recording beam is inclined in a predetermined direction, the reference beam being irradiated in a state that the reference beam is inclined in an opposite direction to the recording beam so that the reference beam crosses with the recording beam at a predetermined crossing angle, the method comprising the steps of converting a Gaussian-beam having an intensity distribution tending to be uniform an intensity distribution of a Gaussian-distribution into a parallel beam having an intensity distribution tending to be uniform, and allowing the parallel beam to travel from the beam modulator to the spatial light modulator as the recording beam, wherein the center axis of the Gaussian-beam is decentered from an optical axis of the beam modulator in a decentering direction including a directional component of the predetermined direction in which the recording beam is inclined or an opposite directional component to the predetermined direction.
Preferred embodiments of the present invention will be described below with reference to the drawings.
As shown in
The hologram recorder A includes a recording-beam optical system for application of the recording beam S to a recording area of the hologram recording medium B at a constant incident angle θ (see
As shown in
The laser beam which comes from the unillustrated light source is a Gaussian-beam which has a light intensity distribution following a Gaussian distribution. This Gaussian-beam is then converted into a parallel beam by the unillustrated collimator lens, and then split by the beam splitter into a recording beam S and a reference beam R. The recording beam S enters the beam homogenizer 1 as a Gaussian-beam. On the other hand, the reference beam R is directed to the recording mirror 10 or to the reproduction mirror 11. It should be noted here that the laser beam (Gaussian-beam) from the light source may first pass the collimator lens and the beam homogenizer before it is split by the beam splitter into a recording beam and a reference beam.
The beam homogenizer 1 is provided by an aspheric lens, converts the incoming Gaussian-beam into a parallel beam which has an intensity distribution tendency toward uniformity, and directs this parallel beam to the spatial light modulator 2 as a recording beam S. The center axis Gs of the incoming Gaussian-beam is decentered from the optical axis 1A of the beam homogenizer 1 by a predetermined decentering amount m in a direction away from the direction in which the recording beam S is slanted with respect to the hologram recording medium B. According to the beam homogenizer 1, it is possible to convert a Gaussian-beam into a beam having uniform intensity distribution with little light loss. It should be noted here that the beam homogenizer 1 in the present embodiment is a non-flipping type which allows the incoming beam to be emitted without inversion around the optical axis; however, a flipping type beam homogenizer may be used which inverses the incoming beam around the optical axis.
The spatial light modulator 2 is provided by e.g. a transmissive liquid crystal device, and modulates the incoming recording beam S into a beam which has a two-dimensional pixel pattern according to the information to be recorded. The recording beam S from the spatial light modulator 2 passes through the zoom lens 3 to be guided to the beam splitter 4 and the recording-beam objective lens 5, and is finally applied to the hologram recording medium B at a predetermined entering angle θ. In this process, the zoom lens 3 flips the recording beam SF around the optical axis by 180 degrees. For this reason, the decentering direction of the Gaussian-beam with respect to the optical axis 1A of the beam homogenizer 1 is opposite to the direction in which the recording beam S is slanted with respect to the hologram recording medium B, as described above. It should be noted here that if the optical system is configured in such a way that the recording beam is not inverted and guided to the hologram recording media, the decentering direction of the Gaussian-beam with respect to the optical axis of the beam homogenizer is the same as the direction in which the recording beam is slanted with respect to the hologram recording media. Such an optical system as the above may be provided, for example, by a combination of a flip-type beam homogenizer and a zoom lens.
The recording mirror 10 and the reproduction mirror 11 are provided by galvano-mirrors, for example. The recording mirror 10 is fixed to a tip of the arm member 20 which is pivotable above the hologram recording medium B. The recording mirror 10 reflects the reference beam R which comes from above into an obliquely downward direction toward the recording area of the hologram recording medium B for recording. The reproduction mirror 11 is fixed to a tip of the arm member 20 which is pivotable below the hologram recording medium B. The reproduction mirror 11 reflects the reference beam R which comes from the side into an obliquely upward direction (not illustrated) toward the recording area of the hologram recording medium B for reproducing. According to the recording and the reproduction mirrors 10, 11 which are provided by galvano-mirrors, it is possible to make fine adjustment on the incident angle of the reference beam R at the recording area.
As shown in
Next, an optical function of the hologram recorder A will be described.
First, if the decentering amount m is zero, i.e., if the Gaussian-beam's center axis Gs is aligned with the optical axis 1A of the beam homogenizer 1, a resulting hologram in the recording area has non-uniform contrast caused by the following three factors:
Specifically, the first factor is the cosine law of illumination, based on the fact that since the recording beam S is oblique to the recording layer 90, there is a significant difference in respective irradiation areas produced by light flux A and light flux B of the recording beam S. As shown in
For example, when it is assumed that the center axis Sc of the recording beam S has an incident angle θ of 35 degrees and α is 23.3 degrees, light flux A through light flux D have geometric optical characteristics as shown in
The AB-direction incident angle in
Irradiation area rate shown in
The second factor relates to relative differences in the polarization direction of the recording beam S and the reference beam R. Referring to
Specifically, considering the polarization directions inside the recording layer 90, the polarization directions can be represented by vectors as shown in
For example, in the case where φ=15.29 degrees, the electric field amplitude of light flux C which contributes to the interference with the reference beam R is represented by cos φ=0.9646 times. Same applies to the electric field amplitude of light flux D. In other words, taking the polarizing direction into consideration, the electric field amplitude of light flux C and light flux D must be increased by multiplication by an inverse number of cos φ, i.e., by a rate of 1.0367; and with this, the intensity distribution of light flux C and light flux D is uniformalized by multiplication by a squared number of the electric field amplitude, i.e., by a rate of 1.0747. Therefore, in consideration of the second factor, it is preferable that the recording beam S should have an intensity distribution of; A:B:C:D=1:1:1.0747:1.0747 with the intensity of light flux A and light flux B as being 1 in order to achieve a uniform intensity distribution.
As the third factor, diffraction efficiency of the recording beam S and the reference beam R must be considered. When it is assumed that θs is the surface incident angle of the reference beam R, θs is the surface incident angle of the recording beam S, d is the thickness of the recording layer 90, λ is wavelength of the recording beam S and the reference beam R, and Δn is refractive-index modulation per page, the diffraction efficiency η is given by the following equation:
With light flux C's and light flux D's diffraction efficiency η being approximately 1, the diffraction efficiency rates of light flux A and light flux B are as shown in
As a conclusion, all of the above-described first through third factors should be taken into consideration in a comprehensive manner, which means by multiplying all the rates for light flux A through light flux D, an optimum intensity distribution is obtained for the recording beam S when A:B:C:D=1.54:1:1.3:1.3.
Based on the above-described points, a simulation was conducted to see how the intensity distribution of recording beam S would change when the decentering amount m of a Gaussian-beam was varied.
As a reference,
Thus, according to the hologram recorder A offered by the present embodiment, the Gaussian-beam which enters the beam homogenizer 1 should only be shifted in an appropriate direction and by an appropriate decentering amount m, and this makes possible to apply the recording beam S of a uniform intensity distribution to the hologram recording medium B with little reduction in the light utilization efficiency, and thereby to record a hologram as a uniform interference pattern.
It should be noted here that the present invention is not limited to the embodiment described above.
The incident angle θ (35 degrees) of the recording beam S, the decentering amount m (0.12 mm) of the Gaussian-beam and so on only represent example values, and the values may be varied appropriately in accordance with specific variations which are made.
It is preferable that the decentering direction of the Gaussian-beam should be as close as possible to the direction in which the recording beam is slanted, as well as to the opposite direction thereto; however, a certain discrepancy from these most desirable direction are acceptable.
Yamakage, Yuzuru, Uno, Kazushi, Yoshikawa, Hiroyasu, Iwamura, Yasumasa, Tezuka, Kouichi
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